1. Field of the Invention:
This invention relates to a method for controlling the telescopic extending and contracting operations of a multistage boom of a crane or the like in an efficient manner.
2. Description of the Prior Art:
Multistage booms generally used on cranes or the like are of the telescopic type, which have their modulus of section reduced gradually toward the fore end of the telescopic boom portions and which are designed to extend firstly a boom portion with the largest modulus of section in an extending operation from the standpoint of the boom strength and to contract firstly a boom portion with the smallest modulus of section in a contracting operation to ensure a rated capacity. Therefore, in controlling the extending and contracting operations of a three-stage boom, for example, the ideal procedure is to extend and contract the telescopic boom portions successively, extending the fore boom portion after the intermediate boom portion is fully extended in the extending operation, and contracting the intermediate boom portion after complete contraction of the fore boom portion in the contracting operation.
With such a multistage boom, it has been the conventional practice to resort to a method of detecting the fully extended state of the intermediate boom portion or the fully contracted state of the fore boom portion by means of a limit switch and switching an electromagnetic valve in a hydraulic control circuit of boom operating cylinders in response to a logic signal with regard to the position of an operating lever, or a method which, in order to preclude errors in the switching operation of the boom operating cylinders, feeds the fluid pressure also to the cylinder of the intermediate boom portion (normally at the stroke end) at the time of extension of the fore boom portion, feeding the fluid pressure even to the cylinder of the fore boom portion (normally at the stroke end) when contracting the intermediate boom portion. Of these conventional methods, the former method is costly since it necessitates providing a take-up reel for winding the electric cables which connect the limit switches to the electromagnetic valve in relation with the telescopic operation of the boom, in addition to the above-mentioned two limit switches for detecting the fully extended state of the intermediate boom portion and the fully contracted state of the distal boom portion, respectively. On the other hand, the latter method incurs a problem in that it likewise requires provision of costly boom operating cylinders and a hydraulic control circuit of complicated construction.